This invention relates to the production of refrigeration during the reduction of pressure of gas withdrawn from a transmission pipeline. More specifically, the invention produces refrigeration with energy recovered when gas at elevated pressure in a transmission pipeline is supplied to a branch or regional pipeline maintained at a lower high pressure.
The gas pressure of transmission or trunk pipelines is reduced at many letdown stations or city gates simply by isenthalpic expansion, i.e., by passage through a reducing valve. Such pressure reduction is a waste of valuable energy.
Two schemes for utilizing the energy available in pipeline gas at letdown control stations are the generation of electrical energy and the liquefaction of natural gas. To produce electrical energy, the pipeline gas is passed through an expansion turbine which drives an electric generator. U.S. Pat. No. 3,360,944 illustrates a process wherein pipeline natural gas is expanded with the performance of work to produce refrigeration utilized to liquefy a portion of the natural gas.
Depending on the location of each letdown control station, the generation of electric energy or the production of liquified natural gas may not be economically attractive. In such case, the conversion of the energy available in the transmission pipeline gas reaching the letdown station to bulk, low-cost refrigeration may be a preferred and valuable alternative particularly where local industries require refrigeration. The frozen food industry, supplies of ice and manufacturers of dry ice are examples of industries which consume large quantities of refrigeration and thus represent potential customers of commercially saleable refrigeration.
Accordingly, a principal object of this invention is to convert energy derived from reducing the pressure of transmission pipeline gas to low-cost refrigeration.
Another important object is to maximize the generation of refrigeration by isentropic expansion of the transmission pipeline gas, i.e., expansion with the performance of work.
These and other objects and advantages of the invention will be evident from the description which follows.